Matter beats out antimatter in experimental echo of creation

Imbalance at Illinois particle accelerator could presage major physics breakthroughs

Less than a trillionth of a second after the Big Bang, another tumultuous event happened. Although the cosmos was born equal parts matter and antimatter, which destroy each other upon contact, matter somehow began to dominate.

TRACKING DECAY The DZero experiment analyzes the decay of particles called B mesons that are created during high-energy collisions between protons and antiprotons at the Fermilab’s Tevatron accelerator. Fermilab

Physicists now have uncovered a new clue about what caused the fortuitous imbalance, which led to the existence of galaxies, planets and people.

The new result is based on eight years of studying the decay of trillions of short-lived particles, called B mesons, that are produced during high-energy collisions at the Fermi National Accelerator Laboratory’s Tevatron particle collider in Batavia, Ill. Scientists on the Tevatron’s DZero experiment have found hints that when B mesons disintegrate, they produce about 1 percent more pairs of muons, a heavy version of the electron, than pairs of the muon’s antiparticle, the antimuon. Physicists refer to the phenomenon as CP violation.

The imbalance, reported at a Fermilab seminar on May 14 and posted online May 18, may bode well for eventually understanding how matter outstripped antimatter in the universe. It also ups the odds that the Large Hadron Collider, the European accelerator that recently superseded the Tevatron as the world’s most powerful atom smasher, will discover new elementary particles or other novel physics.

Although small, the 1 percent surplus is 50 times larger than the asymmetry between matter and antimatter predicted for B meson decays by the standard model of particle physics, notes DZero spokesperson Stefan Söldner-Rembold of the University of Manchester in England.

“It was a goosebump situation,” says Söldner-Rembold of the moment in early May when he and his 500 DZero collaborators realized what they had discovered. “We were very excited because it means there’s some new physics beyond the standard model that has to be within our reach for the asymmetry to be so large.”

Although there’s less than a 0.1 percent chance that the DZero results are a fluke, by the standards of particle physics the results should be regarded as hints that still must be confirmed, cautions theorist Yuval Grossman of Cornell University. Söldner-Rembold notes that the DZero findings are similar to an asymmetry in matter-antimatter production discovered a year ago by another Tevatron experiment, called CDF, but the new results have a much higher certainty.

Theories that might account for the DZero observations include supersymmetry, which assumes that each elementary particle in the standard model of particle physics has an as-yet-undiscovered heavier superpartner, notes theorist Marcela Carena of Fermilab, who is not a member of the discovery team. Other possible theories, she notes, include a model in which gravity and other forces operate in extra, hidden dimensions, and the notion that there’s an additional, fourth family of quarks beyond the three generations (up and down, strange and charm, and top and bottom) that serve as the building blocks of atomic nuclei and some other particles.

In models with a fourth quark family, the presence of new, heavy quarks and their interaction with the three known families could lead to a larger imbalance between matter and antimatter than found in the standard model, Carena notes. In supersymmetry theory, heavy superpartners would play a role similar to that of the heavy quark in creating interactions that might slightly favor the production of matter over antimatter, she adds.

And in theories with extra dimensions, new “messenger” particles — carriers of previously unknown forces — would move in hidden dimensions. These carriers could alter the charge and another property, called flavor, of elementary particles, causing the additional imbalance between matter and antimatter.

“Still, it is difficult to find a theory that can generate this asymmetry without contradicting other experimental results,” Carena adds.

Ulrich Nierste of the University of Karlsruhe in Germany strikes another cautionary note. “The connection of the DZero result to the surplus of matter over antimatter in the universe is vague,” he says. While the DZero result hints at a new source of asymmetry in properties of the B-meson and its antiparticle, the processes that created more particles than antiparticles in the early universe may have involved a very different physical mechanism, Nierste says.

Nonetheless, says Carena, some new source of asymmetry “is needed to explain the matter-antimatter imbalance in the universe and hence our existence.” And for any of the proposed models, “the Large Hadron Collider should have a direct window to observe new particles,” she adds.

One of the smaller experiments at the Collider, designed to examine B mesons, could confirm the DZero team’s findings within a year or two, says Grossman. Larger experiments at the Collider could then hunt for new particles that might be the source of the cosmic imbalance of matter and antimatter and determine their masses, he adds.

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